Ongoing research on this topic focuses on exploring the unique roles of the eosinophilic leukocyte in promoting health and restoring homeostasis. We report on three studies: We first report an important advance related to the quantitative detection of eosinophils in mouse models. Human and mouse eosinophils differ significantly from one another, yet much of our current understanding of eosinophil-associated human pathology has developed from the use of mouse models. While mouse eosinophils can be readily detected by flow cytometric methods, most studies do not document the efficiency of this process compared to direct counting of stained cells. Our intent was to address this knowledge gap by identifying one or more eosinophil-specific antigen profiles that yielded flow cytometric data that was statistically consistent with direct counts. We found that anti-CD193 (anti-CCR3) and anti-CD125 (anti-IL-5Ralpha) antibodies were effective at detecting eosinophils in bone marrow of interleukin-5 transgenic mice, but these antibodies under-reported the percent positive cells. In contrast, anti-Siglec F alone or in combination with anti-CD45 can be used for the quantitative detection of eosinophils in mouse bone marrow and spleen. The antigen profile CD45+SiglecF+CD11c- was effective at detecting eosinophils in the lung as well as bone marrow and spleen, and the results obtained correlated with direct morphometric counts under all conditions evaluated (r-squared =0.98-0.99). This is the first systematic analysis presenting definitive correlations between percent eosinophils detected by cell surface markers and direct counting of stained cells in multiple tissues and at varying degrees of eosinophilia. (Dyer KD Garcia-Crespo KE, Killoran KE, Rosenberg HF. 2011. Antigen profiles for the quantitative assessment of eosinophils in mouse tissues by flow cytometry. J. Immunol. Methods 369:91-97.) The second study utilized our highly-cited protocol for the generation of mouse eosinophils from unselected bone marrow progenitors;this method ultimately led to the identification of an unusual wild-type mouse strain with aberrant eosinophil hematopoiesis. Specifically, we examined the proliferation and differentiation of bone marrow progenitors from inbred Rocky Mountain White (IRW) mice, a strain utilized primarily for retrovirus infection studies. In contrast to findings with BALB/c and C57BL/6 mice, IRW bone marrow cells cannot proliferate or differentiate ex vivo in response to the defined cytokine regimen. Analysis of IRW bone marrow at baseline was unremarkable, including normal numbers of LSK (Lin-Sca-1+c-kit+) and eosinophil hematopoietic progenitors at baseline. Overall, we found diminished expression of transcripts encoding the interleukin-5 receptor subunit alpha (IL-5Ralpha) as well as diminished expression of the receptor on the cell surface;Western blot revealed that IRW bone marrow progenitors over-express the non-signaling soluble IL-5Ralpha decoy protein. Interestingly, IRW mice can generate airway eosinophilia in response to the classic ovalbumin sensitization and challenge Th2 stimulus;however, the response was significantly blunted. (Dyer KD, Garcia-Crespo KE, Percopo CM, Bowen AB, Ito T, Peterson KA, Gilfillan AM, Rosenberg HF;manuscript in revision). Further details of the molecular basis of this hematopoietic discrepancy may be clarified once the genome of this mouse strain is sequenced (Peterson KA, personal communication). The third study focuses on eosinophils and eosinophils secretory ribonucleases and their interactions with respiratory virus pathogens in vivo;the details are included in the Summary Section of Project AI000942 Ribonucleases A in Health and Disease.